In a semiconductor fabrication process, a gate electrode of a MOS transistor is formed by etching a conductive layer that is formed on a semiconductor substrate to a predetermined thickness. Conventionally, polysilicon is used as a gate electrode material. This is because the polysilicon has excellent interfacial characteristics with respect to a gate oxide layer at high temperatures. As the integration level of semiconductor devices increases, the conventional polysilicon gate electrode fails to meet a suitable operation speed and an adequate sheet resistance for the gate electrode. Therefore, a metal gate electrode that is formed by stacking a high melting point metal (e.g., tungsten) on the polysilicon has recently been proposed. Unfortunately, the tungsten is easily oxidized, which results in abnormal oxidation that causes various problems.
With reference to FIGS. 1A-1B and FIGS. 2A-2B, a conventional method of forming a metal gate is illustrated. Referring now to FIG. 1, a gate oxide layer 12 is formed on a semiconductor substrate 10. A polysilicon layer 14, a tungsten layer 16, and a gate capping layer 18 are sequentially formed on the gate oxide layer 12. Although not shown in the drawings, a conductive barrier layer for preventing the polysilicon layer 14 from reacting with the tungsten layer 16 is further formed therebetween. The stacked layers 14, 16, and 18 are etched to form a metal gate electrode pattern 20. Conventionally, an oxidation process is then performed in order to cure an etch-damage (reference number 22) and thereby secure the reliability of the gate oxide layer 12. At this time, a thin oxide layer 12a is formed on a sidewall of the silicon layer 14. Since the oxidation speed of tungsten is far greater than that of silicon, abnormal oxidation (12b) occurs, as shown in FIG. 1B. In this case, a spacer is incompletely formed at the portion of the abnormal oxidation during a subsequent process of forming a sidewall spacer. This results in oxidation therethrough during a subsequent annealing process or creation of a whisker during deposition of a spacer silicon nitride layer.
In order to prevent such abnormal oxidation at the metal gate electrode, a selective oxidation process is widely used. The selective oxidation generally means that after etching a gate, only silicon is oxidized while metal is not oxidized so as to secure the reliability of a gate oxide layer and cure any etch-damage. The selective oxidation is performed to control oxygen and hydrogen gases. Accordingly, H2O and H2 partial pressure is controlled to oxidize silicon selectively.
In a selective oxidation such as wet hydrogen oxidation, the following chemical reaction is controlled to oxidize only silicon.
Reaction EquationSi+2H2OSiO2+2H2  (1)W+3H2OWO3+3H2  (2)
With proper control of H2O and H2 partial pressure, the equilibrium for the first reaction is favored towards the right part of the reaction equation, while the second equilibrium prefers to go to the left. This makes it possible to prevent tungsten oxidation.
However, it is very difficult to control the H2O and H2 partial pressures so as to oxidize only silicon. Accordingly, tungsten is therefore oxidized somewhat, as shown in FIG. 2A. The tungsten oxide layer 12c causes a whisker 24 due to thermal energy in various subsequent annealing steps of the semiconductor fabrication process, as shown in FIG. 2B. The whisker 24 can result in electric short between adjacent gate electrodes.
The whisker 24 is created by an amorphous-phase surface of the tungsten oxide layer 12c and nucleation causing a whisker. Accordingly, surface mobility of the tungsten oxide layer 12c is increased, and is moved to a nucleation position to be crystallized. By repetition of such a procedure, a whisker is created.